1. Field of the Invention
The present invention relates to an ink composition suitably used for inkjet recording, an inkjet recording method, a printed material employing same, a lithographic printing plate obtained using the ink composition, and a process for producing a lithographic printing plate. More particularly, it relates to an ink composition suitable for inkjet recording that cures with high sensitivity upon exposure to radiation, can form a high quality image, has good storage stability, and does not degrade image quality or a substrate due to acid remaining in an image area after forming an image, an inkjet recording method, a printed material employing same, a lithographic printing plate obtained using the ink composition, and a process for producing a lithographic printing plate.
2. Description of the Related Art
With regard to an image recording method for forming an image on a recording medium such as paper based on an image data signal, there are an electrophotographic system, sublimation type and melt type thermal transfer systems, an inkjet system, etc. In the electrophotographic system, a process of forming an electrostatic latent image on a photosensitive drum by electrically charging and exposing is required, and the system is complicated; as a result, there is the problem that the production cost is high. With regard to the thermal transfer system, although the equipment is inexpensive, due to the use of an ink ribbon there is the problem that the running cost is high and waste material is generated. On the other hand, with regard to the inkjet system, the equipment is inexpensive and, since an image is formed directly on a recording medium by discharging an ink only on a required image area, the ink can be used efficiently and the running cost is low. Furthermore, there is little noise and it is excellent as an image recording system.
An ink composition that can be cured by exposure to radiation such as ultraviolet rays and, in particular, an inkjet recording ink (radiation curing type inkjet recording ink) are required to have sufficiently high sensitivity and provide a high image quality. By achieving higher sensitivity, a large number of benefits are provided, such as high curability toward radiation, a reduction in power consumption, longer lifetime due to a decrease in the load on a radiation generator, and prevention of formation of low molecular weight material originating from insufficient curing. Furthermore, higher sensitivity particularly improves the cure strength of an image formed using the ink composition and, in particular, the inkjet recording ink, particularly for the formation of a lithographic printing plate, and high plate life can be obtained.
Such an inkjet system of a type that is cured by radiation such as, for example, ultraviolet rays, generates a relatively low degree of odor and can be applied to recording on a recording medium that does not have quick drying properties or ink absorbing properties; the system has therefore attracted attention in recent years, and an inkjet ultraviolet-curable ink composition employing radical polymerization has been disclosed (ref., for example, JP-A-63-235382, JP-A-3-216379, JP-A-5-214280, JP-B-6-21256, and JP-B-6-62905; JP-A denotes a Japanese unexamined patent application publication, and JP-B denotes a Japanese examined patent application publication). Furthermore, for the purpose of providing an inkjet recording ink that enables an image having high adhesion to a recording medium to be recorded with high sensitivity without spreading, even for a support for which it is usually difficult to carry out direct recording by the inkjet recording method, and where the inkjet recording ink has little skin irritation or sensitization and has high safety, there has been proposed a composition containing a color material and a polymerizable compound comprising a group of specified radically polymerizable acrylate compounds (ref., for example, JP-A-2003-192943 and JP-A-2003-192944). These radically polymerizable inks have excellent curing speed and can form an image without spreading, but they have the problem that the adhesion to the recording medium is degraded due to volume shrinkage during curing.
Because of this, in order to improve the adhesion to the recording medium, cationically polymerizable ink compositions having a low shrinkage ratio during ultraviolet curing have been proposed (ref., for example, JP-A-9-183928). However, these cationically polymerizable inks do not have sufficient storage stability due to a reaction involving an acid generated over time, and this is a big barrier to their actual application. Because of this, as an attempt to improve the storage stability, there has been proposed a technique in which a basic compound or an agent that generates a base on heating is added (ref., for example, JP-A-2003-312121, JP-A-2003-341217, and JP-A-2004-91558), but it has been found that there is the new problem that, since the basic compound inhibits the function of an acid generated on exposure to light, the curing sensitivity of the ink is degraded. Because of this, there has been a desire for the achievement of storage stability and high sensitivity at the same time.